The invention disclosed herein relates generally to monolithic integrated circuits in which an analog output parameter is controlled by a digital signal, and more particularly to digital-to-analog converter and programmable AC resistor and current source designs implemented with field effect transistors in monolithic integrated circuits.
A variety of circuits for achieving an analog output corresponding to a digital input, broadly referred to as digital-to-analog (D-to-A) converters, have been and continue to be required for a wide range of purposes. The trend is toward use of such circuits in increasingly large numbers. Concurrently, there are demands to increase performance, improve reliability and decrease size, power consumption and production costs. Integrated circuit technology has made and continues to make substantial strides toward these general objectives. An increasing number of circuit designs are being reduced to integrated circuit form.
Circuits which have been reduced to integrated circuit form include a variety of D-to-A converters. An example of one such converter is found in U.S. Pat. No. 3,978,473 issued to J. Pastoriza on Aug. 31, 1976. This converter utilizes an integrated circuit switch module including four switch transistors and associated switch control buffering circuitry. The emitter areas of the switch transistors are binarily weighted to provide equal current densities. A fifth reference transistor is provided for adjusting the supply voltage as necessary to maintain constant current through the switch transistors. A number of such switch modules may be combined to construct a D-to-A converter having high bit resolution.
Notwithstanding the foregoing example, prior D-to-A converter designs have generally not optimally adapted integrated circuit technology to the special requirements of D-to-A converters, and have not fully realized the potential of the technology. Many of the designs, such as the one disclosed in the aforementioned patent, utilize bi-polar transistors. Such transistors inherently have offset voltages which must be dealt with, and are essentially unidirectional current devices. Thus, AC conversion with such designs necessitates the use of complex circuitry, and at best, leaves features to be desired.
Various characteristics of bi-polar transistors which are disadvantageous in certain applications are not present in field effect transistors (FETs. Specifically FETs do not inherently operate with an offset voltage, and they are inherently bi-directional. However, at least until recently, field effect transistors have had certain disadvantageous characteristics which made them unsuitable for some applications. Such characteristics included a relatively high "on" resistance, thus limiting the current with which they were useable, a relatively low breakdown voltage, and significant variations in operating parameters of individual FETs. The latter factor affects the accuracy of FET implemented D-to-A converters.
Nevertheless, as shown in U.S. Pat. No. 4,045,793 issued to J. Moench on Aug. 30, 1977, FETs have been advantageously used in integrated circuit D-to-A converter designs. In accordance with this patent various number combinations or groups of field effect transistor switches are coupled between a voltage supply conductor and an analog current output conductor. Each group of field effect transistors is controlled through an input buffer circuit and a control or switching circuit by a separate digital input. Each switching circuit comprises a first field effect transistor connecting the gates of the field effect transistors in a group to one terminal of a current source through the source-drain terminals of the first field effect transistor. The switching circuit also comprises a second field effect transistor connecting the gates of the field effect transistors in a group to a voltage source through the source-drain terminals of the second field effect transistor. The gates of the first and second field effect transistors are respectively controlled from outputs of a series connected pair of inverters. According to the patent, the switching circuitry provides for an analog output which is independent of variations in levels of the digital input signals, and assures that each FET operates in the saturation portion of its current-voltage characteristic to achieve improved accuracy required in many applications.
As disclosed in U.S. Pat. Nos. 3,783,349 issued to J. Beasom on Jan. 1, 1974 and 4,148,046, 4,148,047 and 4,152,714 issued to T. Hendrickson alone or with others on April 3 and May 1, 1979, improved designs have recently been devised for integrated circuit FETs. These designs effectively provide for lower "on" resistance, higher voltage operation, faster response and higher production yields. The improvements are achieved by employing large number of small specially configured and densely arranged cells formed on a semiconductor body.
The applicant has devised a monolithic integrated circuit FET D-to-A converter design which takes advantage of recent improvements in FETs to provide a particularly simple converter and programmable AC resistor network and current source. For present purposes a monolithic integrated circuit is defined as an integrated circuit whose elements are formed in situ upon or within a semiconductor substrate. A large number of ideally identical individual FET cells are utilized. The "on" resistance of the FETs is quite uniform. Sets of the cells operate in parallel, thus averaging out and further reducing any deviations from the predetermined resistance. If resistance value trimming is desired, it can be accomplished with a simple interconnect trim. True AC resistance is provided, with no DC effect. Finally, the device geometry and required production processes are compatible with and facilitate integration with other circuitry, thus permitting fabrication of digital and analog devices on a common substrate.